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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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ablation. Another cause for the clean surface is the abundance of crevasses into which rock fragments would naturally fall. While there is little moraine on the surface, the abundance of black, dirt-laden icebergs that float away from the front of Hubbard Glacier prove that there is much debris in the lower layers; and on the border of each arm there are well-defined lateral moraines, while the northwest arm has also a medial moraine. That the north arm has no medial moraines can only be explained by its rapid motion and the consequent ineffectiveness of ablation, for it is inconceivable that so great a glacier, heading so far back among lofty mountains, can fail to have tributaries of sufficient size to contribute notable supplies of medial morainic d£bris. It is to be noted, however, that close by each lateral moraine, and only a short distance out from it, is a moraine ribbon that probably represents a medial moraine of some tributary, pushed to one side by the dominance of the other arm of the main glacier.
Of the two large arms visible from the sea, the northern is clearly not only the largest but far the most active, and, in fact, dominant in the ice plateau that is made by the coalescing of the two arms. There are several reasons for reaching this conclusion. The first of these is the larger size of the north arm. Even more noteworthy than this is the position of the moraines in the outer ice plateau. The two arms which form the plateau approach each other nearly at right angles, but the dominance of the north arm is made apparent by the fact that its western lateral moraine proceeds in fairly direct course to the fiord, whereas the medial and lateral moraines of the northwest arm bend sharply and extend to the fiord parallel and close to the lateral moraine of the north arm. The northwest arm is far too weak to deflect the lateral moraine of the north arm, but the north arm is strong enough to bend the moraines of the northwest arm at a right angle to the course which they were following up to the time that they came under its influence (PL XLV).
The dominance of the north arm in the terminal ice plateau is also shown by the proportion of ice which it contributes to the terminus. After descending its steepened slope, this arm fans out slightly, and at the glacier front supplies the ice for fully two-thirds of the ice cliff. This conclusion is based upon the distance between the eastern lateral moraine and the moraine which comes down from the northwest arm. We estimate that the width of the ice front dominated by the north arm is fully one-half greater than the width of the north arm where it emerges from its mountain valley over the steepened slope.
In addition to these evidences of the dominance of the north arm, the northwest arm furnishes proof of its own weakness. It has much more moraine on its surface than the north arm, including a medial moraine, and this is interpreted as the result of its slower motion and the resulting greater effectiveness of ablation. Furthermore, the northwest, arm seems to have a buried irregularity of the valley floor, revealed by a slight bulge where the glacier comes out of its hanging valley. The debris-covered ice below the bulge, for which there is no visible source, suggests a ledge near the surface, for this is the only dirty portion of the glacier away from the moraines and may be seen in earlier photographs by Russell and Gilbert. Its character is seen well from near Turner Glacier. Since this area occurs out in the glacier, at some distance from the margin, it is interpreted as evidence that the ice in the northwest arm of Hubbard Glacier is not as thick where it passes over its steepened slope as the north arm is on its hanging valley lip.
The Southeastern Margin.   Partly because of the interesting features which it presents,